JSFiddle - React, Tailwind, and code Playground

HTML

<html>

<body>
    <div id="textarea" contenteditable="true">
        <p>I am on dsvsdvsdv vsdvdsvds sddvdsvds vdsdds sdvdsvdsv vdsvdsvds dsvdsvdsvdsv vsdvdsv sddvsdvds vdsvdsvdsv dsvdefwfs</p>
    </div>
    <script>
        function selection() {
            if (window.getSelection)
                return window.getSelection();
        }

        let textarea = document.getElementById('textarea');

        textarea.addEventListener('mouseup', function () {
            console.log(selection())
            console.log(textarea.innerText.indexOf(selection()))
        })
    </script>
</body>

</html>